1. Field of the Invention
This invention relates generally to synthesis gas conversion and, more specifically, this invention relates to processes for selectively converting synthesis gas to higher alcohols.
2. Brief Description of Related Technology
Large reserves of natural gas or methane are located in remote areas of the world. As oil reserves are depleted, there is great incentive to convert this gas into a commodity liquid fuel. A number of direct methane conversion technologies, such as pyrolysis, oxidative coupling, and direct oxidation exist, but are in the early stages of development. However, there are well-established technologies for the conversion of natural gas into synthesis gas, i.e., a mixture of CO and free hydrogen.
The Fischer-Tropsch process is a well-known synthesis gas reaction for making hydrocarbons. The economics of the Fischer-Tropsch process have been investigated periodically and have generally been found to be unfavorable. The direct synthesis of higher alcohols (i.e., those having 2 or more carbon atoms per molecule) from carbon monoxide and hydrogen has attracted attention because the products are suitable as gasoline extenders and high-octane blending components.
The formation of aliphatic alcohols by the hydrogenation of carbon monoxide is represented by the following equations: EQU 2n H.sub.2 +n CO.rarw..fwdarw.C.sub.n H.sub.2n+1 OH+(n-1)H.sub.2 O[1] EQU (n+1) H.sub.2 +(2n-1) CO.rarw..fwdarw.C.sub.2 H.sub.2n+1 OH+(n-1) CO.sub.2[ 2]
The water-gas shift reaction is closely linked to the alcohol synthesis reaction: EQU CO+H.sub.2 O.rarw..fwdarw.CO.sub.2 +H.sub.2 [ 3]
The hydrogenation of carbon monoxide to hydrocarbons is thermodynamically more favorable than hydrogenation to alcohols; thus, alcohol formation requires selective catalysts in order to minimize hydrocarbon formation.
Catalysts for higher alcohol processes which have reached the commercialization stage or have undergone large-scale pilot plant trials fall into three main categories. They include low temperature methanol synthesis catalysts modified with alkali metals, high temperature methanol synthesis catalysts modified with alkali metals, and modified Fischer-Tropsch catalysts.
Low temperature methanol synthesis catalysts which have been modified for higher alcohol synthesis by the addition of alkali metals usually contain both copper and zinc and may contain oxides of chromium or aluminum. The product of one such catalyst typically contains 50-70 percent methanol depending upon the H.sub.2 /CO ratio of the synthesis gas feed, the balance being C.sub.2 -C.sub.8 alcohols and partially hydrogenated oxygenates. The water content can be reduced to a few percent, while the content of light hydrocarbons is negligible. Typical reaction conditions are 1,500 psig and 520.degree. F. The main shortcomings of this type of higher alcohol catalyst include the presence of a high fraction of methanol in the product, sensitivity of the catalyst to the carbon dioxide level, increased light hydrocarbon production, and deterioration of catalyst activity with time, especially when operated at higher temperatures.
High temperature methanol synthesis catalysts which have been modified with alkali metals to produce higher alcohols usually contain ZnO and Cr.sub.2 O.sub.3 and may also contain oxides of copper. Typical processes of this type operate at H.sub.2 /CO ratios of 0.5-3, a temperature of 625.degree.-800.degree. F., a pressure of 1,300-2,600 psig, and a gas hourly space velocity (GHSV) of 3,000-15,000/hr. The alcohol product is about 70 percent methanol, with the remainder being C.sub.2 -C.sub.5 + higher alcohols and oxygenates. Isobutanol is the principal higher alcohol. At these conditions water can be about 20 percent of the crude product, and hydrocarbon contents are low. The catalysts are quite stable with time. Main drawbacks include the presence of a large amount of methanol in the product, the need to remove large amounts of water, the need to use a synthesis gas feed with a low H.sub.2 /CO ratio, and a high operating pressure.
One example of a modified Fischer-Tropsch catalyst contains MoS.sub.2, CoS, and K.sub.2 O. This catalyst has been reported to yield about 85 percent mixed alcohols, with the remainder as C.sub.1 -C.sub.5 paraffins.
The crude mixed alcohol product of this type of catalyst contains about 50 percent methanol, with the remainder C.sub.2+ alcohols and oxygenates. Ethanol is the major higher alcohol. This catalyst effects a water-gas shift reaction at alcohol synthesis conditions and thus provides a product with less than about 3 percent water. One drawback to this process can be a high yield of light hydrocarbons. The catalyst is believed to require 25-50 ppm H.sub.2 S in the feed gas to maintain acceptable activity.
The preparation of alcohols from carbon monoxide and hydrogen yields a range of alcohol chain lengths as well as linear or branched alcohols. Generally, higher alcohols which form over copper-containing catalysts are branched; those formed over Group VIII metals are predominantly straight chained.
Mixed copper-cobalt alkalized catalysts have been developed by Institut Francais du Petrole for conversion of synthesis gas to higher alcohols. These catalysts generally also contain aluminum, chromium, and zinc. Although these catalysts contain both copper (a component of many methanol synthesis catalysts) and cobalt (a typical Fischer-Tropsch catalyst component), the product distribution is similar to that obtained from a modified Fischer-Tropsch catalyst, i.e., ethanol is the major C.sub.2+ alcohol. A typical such catalyst would yield, on a CO.sub.2 -free basis, 70-80 percent oxygenates and 20-30 percent hydrocarbons. Of the oxygenates, methanol can be 50-70 percent, ethanol 16-25 percent, and the balance other alcohols and partially hydrogenated oxygenates. Such catalysts typically operate at 500 to 600.degree. F., 1000-1500 psig, and a GHSV of 3000-6000/hr, a H.sub.2 /CO ratio of 2 or less, and CO.sub.2 content in the feed gas of less than 3 percent. Drawbacks include the high methanol fraction in the alcohol product and the large amount of light hydrocarbons that are also produced. The performance of this type of copper-cobalt catalyst is especially sensitive to the method by which it is prepared. Large-scale industrial preparation may need very tight controls to ensure an active material.